SOLVED Z-Axis Woes

I'm at the point of the build where I'm calibrating the axes but have had a bit of a problem and I'm afraid I may have done some damage.

I have a v1.1 Pro Hybrid with Titan

I was struggling to get the z-axis zero point to be where it needed to be, the bed wouldn't go high enough up to get to the nozzle and after a few abortive attempts I went back to the beginning. I followed the instruction to 'Restore Failsafe' then carried on.

I got around the z axis not being high enough by moving the IR sensor a little above it's minimum point for the initial setting and after that all was fine. I performed many 'Auto Homes' and got the x and y axes setup nicely.

To double-check the homing I started moving the z-axis down a fair amount but while I was doing this the machine made an awful stuttering noise and I went for the power switch.

Looking at the bed plate, the left was now noticeably higher than the right. A lot of head scratching and I realised that the LCD readout wasn't right for the distance the bed had dropped. When I did the 'Restore Failsafe' I'd somehow missed the setting of the z-steps to 400, double checking and sure enough it was still at 1600.

I found a little strand of swarf on the lower plate that looked like it had come from the coupling so I've dis-assembled the coupling from the motor and the screw but everything appears to be OK. I thought maybe the coupling had slipped on the screw but it seems to be unmarked. The swarf is silver not brass.

When I had the right hand screw out of the machine I had to manually turn the left hand screw to give me more room and I noticed that the right hand motor began to turn on it's own. I know they're wired together so is the left one acting as a generator to drive the right?

The problem I have now is that when I try to move the bed up or down, sometimes the right stepper pauses before starting and it's obviously now right hand side low.

I'm sure one motor can drive another, this is a technique used to drive all sorts of encoders, depth measurement etc. At least this indicates that both motors are working in some way.

You need to remove the coupling and screws rod from the left motor so both left and right are completely removed from the screwed rods, then you can do some testing.

Both Z motors are connected to the same driver board one reason why one drives the other.

If one motor moves and the other hesitates and then moves then the driver board is doing something so not completely broken.

So you now have to sort out if the hesitation is electrical/electronic or mechanical... the piece of swarf must have come from either the screwed rod, or coupling or motor shaft.

With all the parts dismantled and the motors just sitting there with the shafts visible check all items to see where the swarf came from. It could have come from the coupling/motor area, less likely from the fitting in the bed as this is brass or bronze. It presumably was graunched out as the bed stopped but the motor kept turning.

Mechanical test are best done with the motor wiring disconnected from the Rumba board. Remove the wires, noting where they were connected and try moving the shaft of each motor by hand. If the one that hesitates has more mechanical resistance then it was probably damaged when the bed stopped and the motor continued to rotate.

If the mechanical test shows one with considerably more resistance to movement then it is academic whether you do an electrical test too, but what you could do is wire just one motor at a time to the Rumba and use OctoPrint under the control tab to "move the bed" and see what happens i.e. if one of the motors hesitates and the other does not.

If there is any hesitation then if this corresponds with the increased mechanical resistance then you have the problem.

Also if one motor works fine and the other not when tested individually then the driver board is probably ok.

At this point you have to decide if you simply cut your losses and order another stepper motor, you could include in the order another driver board (remember anti-static precautions if you change it)

Other checks could be wiring and tightness of connections and the driver board voltage adjustment, but these are less likely to be the problem if one motor works fine and the other not.

You probably want to set Z stepper reference voltages a bit higher than the minimum, the motors are wired in parallel and the stepper setting doesn't take this into account so you essentially are driving them with half the current limit of the others.

I disconnected the screws from both sides, lifted the bed and supported it out of the way.

I blu-tacked an allen key to the top of each coupling and tried to move the z-axis. The left-hand motor rotated as expected but the right-hand just sat there and stuttered. When I changed the movement to 1mm, the stuttering motor then started rotating the wrong way!

I increased the voltage but it didn't help.

I tried swapping the motor cables and everything is now OK!

From my aeromodelling, a stuttering brushless motor usually means a wiring fault. I can only think that when I looked at the right hand motor yesterday, I didn't re-seat the connection to the motor correctly.